What is the standard by which you judge whether a photo has been over-manipulated? Do you judge it against the reality of the original scene that was photographed? Certainly that makes sense, but it’s hard to judge that objectively if you weren’t present when the photograph was captured. Instead, it’s common to judge the modifications against the original, unaltered photograph that the camera captured—despite the fact that the camera itself tends to change the scene in subtle ways due to lens distortion, sensor design, and internal processing algorithms. A new technology trend known as computational photography, however, is changing how we approach image capture, and this may have implications for how we judge what is manipulated.
Welcome to the Fourandsix blog, where you’ll find tips on image forensics techniques and commentary on issues relevant to photo tampering and the responsible use of imaging tools.
Grooves made in a gun barrel impart a spin to the projectile for increased accuracy and range. These grooves introduce distinct markings to the bullet fired, and can therefore be used to link a bullet to a specific handgun. In the same spirit, imperfections in manufacturing lead to slight deviations between the precise amount of light that strikes a camera sensor and the recorded pixel values. These deviations vary from camera to camera (even of the same make and model) and can therefore be used for camera ballistics — linking an image to a specific digital camera.
A power grid delivers power at a specific frequency termed the Electric Network Frequency (ENF). The ENF varies from country to country, but each power grid strives to deliver power at a fixed frequency. Due to varying loads on the power grid the ENF fluctuates slightly, typically by less than a fraction of a percent. These fluctuations affect digital recording devices enough so that the fluctuations can be seen in the recording. Recent advances have shown how ENF fluctuations can be used to authenticate digital recordings, particularly audio recordings.
The JPEG format employs a lossy compression scheme which sacrifices image detail in order to reduce the stored file size. Once an image is compressed this image detail is irreversibly lost. While this loss of information is often problematic for a forensic analyst, it can also be used to determine if an image has been altered from the time of its recording.
Don’t do it. Seriously, the Photoshop Liquify command can be a useful creative tool, but I’m not aware of—nor can I imagine—any practical use of the tool that would maintain the truthfulness of an image. (Of course, I’m constantly amazed by the unexpected uses people find for some Photoshop tools, so feel free to let me know in the comments if I’m mistaken.) In fact, there’s not much for me to say about the use of Liquify in a truthful editing workflow, but I have been thinking lately about just how big an impact this single feature has had on the “extreme” retouching we see all around us these days.